Low density polyethylenes (LDPEs) made using a conventional high-pressure process, linear low density polyethylenes (LLDPEs) produced using a Ziegler-Natta catalyst or a metallocene catalyst, other polyethylene polymers made using a metallocene catalyst in a gas phase process, and blends and articles made therefrom, are generally known in the art. While such polyethylenes are sometimes preferred because they provide relatively low-cost solutions to a number of needs, their properties render them less desirable than other polyethylenes for a number of applications. For example, LLDPE and LDPE films generally cannot be produced in high-stalk bubble blown film lines due to a lack of melt strength, and therefore cannot be produced with balanced machine direction (MD)—transverse direction (TD) shrink properties.
Some metallocene LLDPE films provide excellent mechanical properties such as impact and tear resistance but have poor bubble stability during film blowing. Previous attempts to remedy the situation by the addition of long-chain-branched PEs such as LDPE or other branched PEs (U.S. Pat. No. 6,870,010) have resulted in decreased mechanical properties. Some of the blown films blended with branched PE additives additionally suffered from poor optical properties, e.g., the existence of gel particles. Moreover, typical metallocene catalyzed polyethylenes (mPE) are somewhat more difficult to process than LDPE made in a high-pressure polymerization process. Generally, mPEs (which tend to have narrow molecular weight distributions and low levels of branching) require more motor power and produce higher extruder pressures to match the extrusion rate of LDPEs. Typical mPEs also have lower melt strength which, for example, adversely affects bubble stability during blown film extrusion, and they are prone to melt fracture at commercial shear rates. On the other hand, mPEs exhibit superior physical properties as compared to LDPEs. In the past, various levels of LDPE have been blended with mPE to increase melt strength, to increase shear sensitivity, i.e., to increase flow at commercial shear rates in extruders, and to reduce the tendency to melt fracture. However, these blends generally have poor mechanical properties as compared with neat mPE. It has been a challenge to improve mPEs processability without sacrificing physical properties.
U.S. Pat. No. 6,870,010 describes a low density substantially linear polyethylene composition, including some that have: (a) a density of 0.935 g/cc or less; (b) a Haze of 10% or less, (c) a Dart Impact of 100 g/mil or more, (d) an average overall long chain branching index of 0.95 or more; and (e) a slice long chain branching (SLCB) index of 0.85 or less for any portion of the composition having a molecular weight of 100,000 or above.
U.S. Pat. No. 7,951,873 discloses blends of linear low density polyethylene copolymers with other linear low density polyethylenes or very low density, low density, medium density, high density, and differentiated polyethylenes. It also includes articles produced from the linear low density polyethylene and polyethylene blends described therein.
International Patent Application WO 2014/042898 provides ethylene-based copolymers, particularly ethylene-based polymers having about 80.0 to 99.0 wt % of polymer units derived from ethylene and about 1.0 to about 20.0 wt % of polymer units derived from one or more C3 to C20 α-olefin comonomers; the ethylene-based polymer having a local maximum loss angle at a complex modulus, G*, of 2.50×104 to 1.00×106 Pa and a local minimum loss angle at a complex modulus, G*, of 1.00×104 to 3.00*×104 Pa. This patent application also includes articles, such as films, produced from such polymers and methods of making such articles.
International Patent Application WO 2014/088827 discloses polyethylene compositions comprising one or more ethylene polymers and one or more HDPE modifiers, in particular, this publication further relates to polyethylene blends comprising one or more ethylene polymers and one or more HDPE modifiers, wherein the modifier has: 1) a density of greater than 0.94 g/cc; 2) a Mw/Mn greater than 5; 3) a melt index (ASTM 1238, 190° C., 2.16 kg) of less than 0.7 g/10 min; and 4) a g+vis of 0.96 or less. Other background references include WO 2013/052273 and WO 2014/003923.
As discussed above, while a lot of efforts have been made to blend LDPE/LLDPE with other ethylene polymers, it is difficult to create one desired blend to combine favorable melt processing properties and optical properties in films made therefrom. There remains a wide need to find an alternative blend partner for LDPE/LLDPE to contribute to both desired toughness and optical properties and improve processability for the films that are produced therefrom. Applicants have found that such objective can be achieved by using an inventive polyethylene polymer catalyzed by a zirconium-based metallocene catalyst, which exhibits remarkably high melt strength resulting from presence of long chain branching and its low melt index. The inventive polyethylene polymer can deliver improved melt strength and strain hardening behavior to its blend with LDPE/LLDPE, leading to simultaneous improvement in both toughness and optical properties of films. Therefore, the inventive composition can be well suited for use in film applications requiring a good balance between toughness and optical properties and can be effective in improving processability in film extrusion process so as to be qualified as a desired candidate to replace conventional blends of LDPE/LLDPE used in shrink films and high-stalk film extrusion process, as well as for applications beyond films, including sheet extrusion, foam and pipe, etc.